The advantages of free-flowing rubber powders containing fillers compared with the conventional bale mixture have often been described in the literature (see, for example, U.S. Pat. No. 4,250,082). Mainly carbon blacks at all levels of activity are commonly used as fillers in the rubber industry. However, the use of light fillers, preferably the highly active silicic acid (SiO.sub.2), is also known.
Most prior art processes for the production of rubber powders with silicic acid as filler occur by treating mixtures of rubber latex and water glass (i.e., sodium silicate) with acids or acidic salts in the presence of large amounts of neutral electrolytes, removing the resultant precipitate, washing it salt-free and drying it. The main disadvantage of this so-called in-situ precipitation procedure is that large amounts of waste water, loaded with organic materials and electrolytes, are accumulated. A further disadvantage is that the rubber powders accumulate in very fine particles (see No. DE-PS 20 30 172, column 1, lines 63 and 64) and thus have a tendency to give off undesired dust during processing.
The disadvantages of in-situ precipitation were able to be overcome through earlier work, but with the limitation that sufficient filler degrees, which are significant for the direct processing of rubber powders with silicic acid as fillers, could not be achieved.
By the process in U.S. patent application Ser. No. 020,928 rubber powders were successfully produced with silicic acid as a filler which exhibited the desired rubber-silicic acid ratio and, at the same time, exhibited good powder engineering qualities relative to free-flowing, dust-free and storage stability properties. The process is, however, limited to the use of silicic acids with a specific grain size distribution.